Accumulation of polyunsaturated fatty acids by cladocerans: effects of taxonomy,temperature and food

1. Zooplankton are important in transferring dietary nutrients, including polyunsaturated fatty acids (PUFA), up through aquatic food webs. 2. We tested the hypothesis that the taxonomic composition of zooplankton affects the retention and subsequent transfer of PUFA from upwards through the food web. Using laboratory experiments, we investigated dietary PUFA accumulation and bioconversion capacities of six cladoceran species (Ceriodaphnia sp., Daphnia longispina, Daphnia magna, Daphnia pulex, Scapholeberis mucronata and Simocephalus vetulus) fed on two diets (Scenedesmus obliquus and Cryptomonas sp.) that differed in their PUFA profiles. We performed experiments at two different temperatures (14 and 20 °C) to assess the role of temperature in the trophic transfer of PUFA. 3. There was little variation in the concentrations of PUFA in these cladocerans which were controlled by dietary PUFA supply. Moreover, as expected, the concentrations of PUFA in all cladoceran species were higher at low temperature. 4. However, even if the composition of PUFA in the cladoceran species generally corresponded to that in their diet, preferential accumulation of some PUFA was recorded in all these taxa. When fed on a highly unsaturated fatty acid‐deficient diet, all the cladocerans showed some ability to convert C18‐PUFA into arachidonic acid and eicosapentaenoic acid. Interspecific variation in the ability to accumulate and bioconvert PUFA in cladocerans was more pronounced at low temperature (14 °C) for both diets. 5. Our results strongly suggest that in heterogeneous habitats with food partitioning between co‐existing cladocerans, foraging behaviour may affect the transfer of PUFA more strongly than interspecific variation in accumulating and/or bioconverting dietary PUFA.     

Experimental study of indirect effects of fish on the demographic parameters of cladoceran species under eutrophic conditions

Indirect effects of fish on the demographic parameters of cladoceran species were studied under eutrophic conditions. Laboratory experiments were performed with water from control and fish mesocosms to avoid the direct impact of fish predation. In the experiments with the water from the fish mesocosms, fish indirectly negatively affected the demographic parameters of large cladocerans (Daphnia magna and D. pulicaria) due to the enhanced abundance of blue-green algae in the phytoplankton. However, small Ceriodaphnia quadrangula and littoral species Simocephalus vetulus did not respond to the presence of blue-greens. Due to this mechanism, the total abundance of cladoceran species can be sustained during the development of blue-green algae because large and small bodied species differ in their resistence to high concentratons of blue-green algae. Fish chemical signals (kairomones) did not influence the demographic parameters of any cladoceran species.     

A laboratory study on the demography and competition of three species of littoral cladocerans from Lake Huetzalin,Xochimilco, Mexico

Lake Huetzalin (Xochimilco) is a shallow waterbody in Mexico City with a dominance of the littoral cladocerans Simocephalus vetulus, Alona rectangula, and Pleuroxus aduncus. Here, we present data on the life table and population dynamics of these three cladoceran species on two types of diets (alga Scenedesmus acutus and detritus). We also analyzed the effect of competition on population growth using Scenedesmus, which proved to be the better diet than detritus. We hypothesize that feces of the larger cladoceran species (S. vetulus) could be a food source for the smaller taxa (A. rectangula and P. aduncus). Experiments were conducted in 100 ml beakers with 50 ml of medium in each at a temperature of 23 ± 1°C. We studied the demography of three test species using two food types with four replicates for each treatment in the life table experiments. Based on the data thus obtained we further studied the competition between the cladocerans on a diet of Scenedesmus. The maximal growth rates obtained for P. aduncus, S. vetulus, and A. rectangula fed Scenedesmus acutus were: 0.1, 0.3, and 0.5 d⁻¹, respectively. Among the test species, S. vetulus had the highest survivorship of 77 days when cultured on detritus and P. aduncus the lowest of 20 days on Scenedesmus. The gross reproductive rate (offspring female⁻¹ lifespan⁻¹) ranged from 6 neonates in Pleuroxus to 160 in Simocephalus per female. From the competition experiments conducted using alga as the diet, we observed that A. rectangula and, especially P. aduncus, had higher growth rates in the presence of S. vetulus than in the controls. This facilitation could be one of the reasons for higher species diversity in the littoral of water bodies.     

Characteristics of six cladocerans in relation to ecotoxicity testing

Investigation was made to compare some biological characteristics relevant to ecotoxicity testing among six cladoceran species, including Daphnia magna, Daphnia carinata, Daphnia pulex, Ceriodaphnia quadrangular, Bosmina longirostris, and Simocephalus vetulus. The results show that D. carinata had advantages over other cladoceran species for being used as a test organism, particularly for ecotoxicological assessment of aquatic environments in tropical and subtropical areas. D. carinata had similar body size and total number of offspring per female to D. magna. However, D. carinata was more sensitive to the reference toxicant and had much shorter reproduction cycle than D. magna. D. carinata had similarity to D. pulex, C. quadrangular, S. vetulus and B. longirostris in terms of sensitivity to the reference toxicant and length of reproduction cycle. However, D. carinata was much larger in size and produced much more offspring per female than any of D. pulex, C. quadrangular, S. vetulus and B. longirostris. Among the investigated cladocerans, only the neonates (2- and 4-day-old) of D. carinata exhibited phototaxis that was sufficiently remarkable. The low among-generation variation in phototaxis index (I_p) of D. carinata and the close relationship between I_p and the concentration of the ISO standard toxicant (K₂Cr₂O₇) appears to suggest that I_p can be used as an excellent test endpoint for ecotoxicity testing.     

Effects of individual variability on the outcome of competition between cladoceran species

Simulation modeling was performed to study the impact of individual variability on the population dynamics of particular cladoceran populations and the outcome of interspecific competition at low and high temperatures. The study was performed on one species of small size (Ceriodaphnia quadrangula O.F. Müller) and one species of large size (Simocephalus vetulus O.F. Müller). Laboratory experiments were performed to estimate the average values of principal demographic parameters of these species and their coefficients of variability at two temperatures, 18 and 27°C. Population dynamics of the two species in mono- and mixed cultures was simulated with and without accounting for individual variability using available published data and our own data on demographic parameters. Individual variability was shown to help populations survive unfavorable trophic conditions and to give competitive advantage over populations in which all individuals of the same age are identical. Individual variability decreased with rising temperatures due to probable acceleration of physiological processes.     

Changes in prey preferences of dragonfly naiads of Rhionaeschna multicolour (Hagen, 1861) (Odonata: Aeshnidae) in the presence and the absence of macrophytes

We offered prey consisting of the rotifers, cladocerans, copepods, amphipods, dipterans and ephemeropterans to the pre-starved and sorted in three size groups of naiads of Rhionaeschna multicolour (Hagen, 1861) in the presence of the macrophyte Egeria densa Planchon and without it. The naiads consumed up to 25 prey items or in terms of biomass (wet weight) up to 7 mg within 2 h. Biomass intake increased with increasing predator's size. Prey consumption was significantly lower in presence of plants. The cladocerans Ceriodaphnia dubia Rihcard, 1894, Daphnia mendotae (Taylor and Hebert, 1993) and Ilyocryptus sp. were consistently preferred by the dragonfly naiads while Simocephalus vetulus (Müller, 1776), Alona guttata Sars, 1862 and Scapholeberis kingi Sars, 1888 were avoided with and without macrophytes. This study suggests that dragonfly naiads structure the zooplankton community by selective feeding.     

Interactions between the Anomopod Cladocerans Ceriodaphnia Dubia,C. Cornuta,Simocephalus Vetulus and S. Serrulatus,the Aphanoneurid Worm Aeolosoma sp., and the Fish Skiffia Lermae: Predation or Competition,or Both?

We tested the reciprocal effects of water conditioned by exudates of Aeolosoma sp. (Aphanoneura, Oligochaeta) and fish (Skiffia lermae: Goodeidae) and by the presence of Aeolosoma sp. on the population dynamics of four species of anomopod cladocerans (C. cornuta, C. dubia, S. serrulatus and S. vetulus) at 25 °C for 26 days using Chlorella vulgaris(1 × 10⁶ cells ml^–1) as a basic food for all. We found that, regardless of treatment, C. cornuta and C. dubia had a long initial phase of slow growth, followed by a rapid increase after the second week and that, regardless of treatment, C. dubia had a higher rate of population increase per day than C. cornuta. S. vetulus had a longer lag phase than S. serrulatus. Fish-conditioned water resulted in growth inhibition in S. serrulatus, but had no effect on S. vetulus, while Aeolosoma-conditioned water inhibited growth of S. vetulus but not that of S. serrulatus. Our results suggest a scale of responses of cladocerans to exposure to fish- and worm-conditioned water and to the presence of worms, ranging from a mild stimulation to no effects to inhibited growth. Kairomone-effects on body size of C. cornuta were not significant, but beak length was strongly influenced, and more by fish than by worm kairomones. In S. vetulus, differences in size were not significant. However, individuals grown in the presence of worms had higher biomass while those grown in the presence of fish and worm kairomones showed a decreased weight. Effects of worm exudates and of their live biomass on cladocerans were thus opposite in Simocephalus. In Ceriodaphnia dubia, in contrast, they were additive. Aeolosoma was depressed by all four cladocerans, although worms tried to penetrate the valves of the cladocerans to feed on the tissue inside. In doing so, they suffered significant casualties, especially from blows of the powerful post-abdomen of Simocephalus. The nature of the cladoceran-worm interaction is therefore far from simple: in addition to the 'chemical communication' that is present, it has a predatory component (worms trying to feed on cladocerans) but the reverse might also be true (cladocerans filtering out tissues of disrupted Aeolosoma). Since both worms and cladocerans feed on algae, exploitative competition also seems to be involved. In this, the worms appear to be the inferior partner, although none of the experiments lasted long enough to drive any of the competing partners to extinction.     

Evaluation of respiration parameters in a few generations and clones of <Emphasis Type="Italic">Simocephalus vetulus</Emphasis> (Crustacea,Cladocera) under controlled conditions

Cladocera use their thoracic filter limbs, animated by rhythmic and persistent movements, for food collecting and gas exchanges. Several researches have been performed on Daphnia magna in natural environments and in laboratory to identify the adaptations developed toward a decrease or a lack in oxygen. However, no experiment was performed on cladocerans living exclusively in littoral areas, which need efficient adaptations to survive an always changing environment especially regarding oxygen concentration. These changes require from organisms that they spend additional energy in order to satisfy their need in oxygen. The present laboratory experiments were based on a typical inhabitant of littoral areas, Simocephalus vetulus, and on individuals which came directly from natural environments. Respiration rate and NADH fluorescence in filter limb muscles, as a marker of tissue oxygenation, were analysed on 20 families distributed over five generations, all acclimated and cultured under constant food supply conditions. The results underlined differences among generations especially due to body dimensions, and consequently to filter dimensions. As opposed to our former hypothesis, small and young individuals appeared to be more active and thus display a more efficient respiration than big and old Simocephalus.     

Summer zooplankton dynamics in the limnetic and littoral zones of a humic acid lake

The limnetic and littoral zooplankton were studied during summer 1989 in Triangle Lake, a humic acid (pH 4.9) bog lake in Ohio, USA. The limnetic zooplankton showed low species richness and biomass, and dominance by the rotifers Kellicottia bostoniensis and Polyarthra vulgaris. In the littoral, species richness and biomass were markedly higher, and the crustaceans Alona guttata, Ceriodaphnia quadrangula, Chydorus sphaericus, Simocephalus serrulatus and Tropocyclops prasinus, and the rotifer Ascomorpha ecaudis were the dominants. The extreme rotifer dominance and lack of cladocerans in the limnetic zone were likely the result of Chaoborus predation. A pronounced mid-summer decline in cladoceran biomass in the littoral was likely due to predation by T. prasinus and Utricularia (bladderwort).     

Lifetable demography of four cladoceran species in relation to algal food (Chlorella vulgaris) density

Algal food density is known to influence life history variables of cladoceran species. It is not, however, well established whether both littoral and planktonic cladocerans show similar trends when exposed to increasing food concentrations. In the present work, we studied the life table demography of four cladoceran species (Ceriodaphnia cornuta, Moina macrocopa, Pleuroxus aduncus and Simocephalus vetulus) in relation to three algal food concentrations (low: 0.5 × 10⁶, medium: 1.5 × 10⁶ and high: 4.5 × 10⁶ cells ml^–1 of Chlorella vulgaris) (in terms of carbon content, these were equivalent to 0.15, 0.45 and 1.35 g ml^–1, respectively) at 25 °C. In general, for all the tested cladoceran species, values of average lifespan, gross reproductive rate, net reproductive rate, generation time and the rate of population growth were higher at lower food concentrations. Furthermore, high food concentration resulted in a negative population growth rate (mean ± standard error: –0.091 ± 0.026) for P. aduncus. The highest population growth rate (0.602 ± 0.014) was recorded for M. macrocopa at low food density. S. vetulus had the longest average lifespan (40 ± 1 d) while M. macrocopa had the lowest (5 ± 1 d). C. cornuta showed better performance at medium food concentration. We conclude that among the algal concentrations used here, 0.5 × 10⁶ – 1.5 × 10⁶ was beneficial not only to the planktonic species but also to the littoral P. aduncus and S. vetulus while 4.5 × 10⁶ cells ml^–1 was unsuitable for all the cladocerans tested.     

An energy budget for Simocephalus vetulus (O. F. Muller) (Crustacea : Cladocera)

We discuss the energetics of a cladoceran, Simocephalus vetulus at different temperatures (8.0 ± 1.0, 15.0 ± 1.0, 21.0 ± 1.0 and 28.0 ± 1.0 °C) and food (Chlamydomonas sp.) concentrations (25 × 10³, 50 × 10³, 75 × 10³ and 100 × 10³ cells ml⁻¹). Increase in temperature accelerated ingestion and, to some extent, oxygen consumption. The study revealed a high reproduction efficiency in S. vetulus. Net growth efficiency (ECI) was higher (13.17–41.18%) in pre-adults than in adults (2.71–8.40%). The assimilated energy (A) increased with increasing food concentration at all temperatures. Assimilation efficiency (AD) decreased with increasing food concentrations. The energy used for growth (P) was nearly constant at all food levels because the egested energy increased and assimilation efficiency decreased as food concentration increased.     

The effect of temperature,irradiance and animal size on incorporation rates of Simocephalus vetulus

Carbon incorporation rates of Simocephalus vetulus were measured to study the effects of the physical state of the animals, size of the animal, varying temperature and light conditions. Physical state of the animal showed little effect on incorporation rates after the first hour. Incorporation rates increased in proportion to the third power of animal size. Experimental animals collected at temperatures of 12, 20 or 25°C fed maximally at 10, 15 and 25°C respectively, when subjected to a feeding temperature range of 5 to 30°C. We have interpreted this as an indication that S. vetulus is able to acclimate and incorporate maximally at various temperatures after prolonged exposure to that temperature. When fed over an irradiation range of 0 to 4.68 × 10^–3 cal cm^–2 s^–1 incorporation rates were indirectly proportional to irradiance. This suggests a response to decreased irradiance in the weedy, littoral habitat of these animals.     

Adaptive microevolutionary responses to simulated global warming in Simocephalus vetulus: a mesocosm study

Although several studies suggest the occurrence of microevolutionary responses that may allow local persistence of populations under global warming, rigorous experimental proof is lacking. Here, we combined the realism and rigid, replicated experimental design of a large‐scale mesocosm study where populations of the zooplankter Simocephalus vetulus were exposed for 1 year to different global warming scenarios with a life table experiment under laboratory conditions at three temperatures that eliminated confounding, nongenetic factors. Our results provide solid proof for a rapid microevolutionary response to global warming in both survival and the subcomponents of individual performance (age at reproduction and number of offspring), which may allow populations of S. vetulus to persist locally under predicted scenarios of global warming. Such microevolutionary responses may buffer changes in community structure under global warming and help explain the outcome of previous mesocosm studies finding only marginal effects of global warming at the community level.     

Development of a zooplankton community including <Emphasis Type="Italic">Heterocope saliens</Emphasis> Lillijeborg in an experiment with hydrophytes

The influence of the submerged plants Ceratophyllum demersum L. and Elodea canadensis Michx. and the floating plant Hydrocharis morsus-ranae L. on the species composition and quantitative parameters of a zooplankton community was studied experimentally. Among submerged vegetation, the development of the predaceous calanid Heterocope saliens Lilljeborg was suppressed. An increase in the number of zooplankter species was observed in all experimental ecosystems with hydrophytes. The species similarity of zooplankton was higher between communities with plants of the same ecological group than with plants of different groups. The highest average zooplankton biomass, as determined by the abundance of Daphnia longispina O.F. Müller and Simocephalus vetulus (O.F. Müller), was observed in experiments with Elodea. The highest average abundance over the experimental period was recorded among Ceratophyllum, where the abundance of Rotifera, chydorids, and copepods common in hydrophyte beds was higher than in other versions of the experiment.     

Biotic relations affecting species structure in zooplankton communities

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community. Handling editor: S. I. Dodson     

Growth and reproduction of three cladoceran species from a small wetland in the south-eastern U.S.A.

SUMMARY 1. Growth, reproduction and life‐history parameters were measured for three cladoceran species from a small south‐eastern wetland, U.S.A. Simocephalus serrulatus, Diaphanosoma brachyurum and Scapholeberis mucronata juveniles were reared at temperatures between 10 and 25 °C on natural food resources. 2. Growth rate increased with temperature and decreased with individual size for all three species. Maximum somatic growth rate was higher for Simocephalus (49–72% day^?1) and Diaphanosoma (21–91% day^?1) than for Scapholeberis (11–45% day^?1). Multiple regression equations were developed which predict temperature‐ and mass‐specific growth rates for each species. 3. Scapholeberis egg production was positively related to temperature; however, maximum egg production occurred at intermediate temperatures for Simocephalus and Diaphanosoma. Mean cumulative egg production was higher for Scapholeberis (28–92 eggs per female) than for Simocephalus (18–25 eggs per female) and Diaphanosoma (1–41 eggs per female), and was related to differences in reproductive strategy and survival. 4. Survival was inversely related to temperature in most cases. For all three cladocerans, the intrinsic rate of increase (r) and net reproductive rate (R₀) increased with temperature, whereas generation time (G) decreased. Greater egg production by Scapholeberis compared with the other two cladocerans was consistent with higher R₀ values for Scapholeberis at any given temperature. Although r was very similar among species, G was typically longer for Scapholeberis than for Simocephalus and Diaphanosoma. 5. This analysis provides basic information about the population parameters of these coexisting wetland species, and the growth rate models can be applied to field data to determine production dynamics.     

Cyanobacteria consumption by cladocerans: a case study on facilitation

Eutrophic systems are stressful for zooplankton species, especially for small organisms (<?1000 μm) that are inefficient in consuming large colonies or filaments of cyanobacteria. Certain mechanisms, however, enable organisms to coexist in spite of the stress related to poor food quality or manageability of the diet. The present work suggests that coprophagy is recurrent behavior in cladocerans that may facilitate the survival and growth of some species in eutrophic systems. We chose three clones of Moina macrocopa that inhabit eutrophic systems. The species selected as possible facilitators were the cladocerans Simocephalus vetulus and Daphnia similis, and the ostracod Heterocypris incongruens, because they are cosmopolitan, have high rates of cyanobacterial filtration and in some cases coexist with Moina. The design used for the demographic experiments of the three clones of M. macrocopa was based on the source of nutrition and consisted of a control diet of Scenedesmus acutus (1?×?10⁶ cells ml), a diet of undigested cyanobacteria (10?×?10⁶ cells ml), and this same diet after digestion by D. similis, S. vetulus or H. incongruens. The excreta of the cladocerans was a deficient diet for Moina, whose populations decreased by more than 1 individual per day. On the contrary, fecal pellets of the ostracod were a diet that allowed Moina to have positive population growth (0.3 day^?1). Our results help to explain the high densities achieved by cladocerans in eutrophic tropical water bodies where cyanobacterial blooms are normal.     

Responses of rotifers and cladocerans to Microcystis aeruginosa (Cyanophyceae): A demographic study

The response of selected rotifers and cladocerans to Microcystis aeruginosa, offered as colonies and single cells, was compared to that on a diet of Chlorella vulgaris using the life table demography approach. The test zooplankton species were Simocephalus vetulus, Daphnia carinata, Moina macrocopa, Scapholeberis kingi, Ceriodaphnia cornuta, Brachionus calyciflorus and Hexarthra mira. To detect the development of resistance to toxins from Microcystis in zooplankton, in addition to the laboratory cultured strain of Ceriodaphnia cornuta, another strain of the same species was also used (designated as C. cornuta 2, this was collected from a pond containing Microcystis and cultured in the laboratory on Chlorella for a few weeks prior to experimentation). Experiments were conducted at 20°C and 30°C. Survivorship was high on Chlorella in most species but low on diets of Microcystis. Except for C. cornuta 2, S. kingi and S. vetulus, all other test species were adversely affected by Microcystis. The ability to utilise Microcystis improved at 30°C in M. macrocopa, D. carinata and H. mira. The longest mean lifespan was recorded for C. cornuta 2 (25.3 ± 4.86 d) and the lowest for B. calyciflorus (0.58 ± 0.05 d). The highest net reproductive rate was observed for C. cornuta 1 (44.9 ± 4.88) and the longest generation time of 26.6 ± 2.13 d for S. vetulus. Among the cladocerans that showed positive values of population growth rate (r), M. macrocopa had the highest of 0.96 ± 0.04 per day.     

Development,Reproduction and Growth Pattern of two Coexisting,Pond Dwelling Cladocerans

Laboratory investigations on life history parameters of 2 coexisting cladocerans (Daphnia obtusa. Moina brachiata) from a nearly temporary pond in South Germany revealed that the species have different temperature tolerances and temperature optima. D. obtusa experienced the highest reproductive success at 15 and 20 °C and was able to survive and to reproduce at 2 °C but died at 30 °C. The reproductive success of M. brachiata was highest at 25 and 30 °C and the species could not withstand temperatures <15 °C and ≥35 °C. At temperatures between approximately 20 and 25 °C, where both cladocerans coexisted in nature, M. brachiata showed a faster population growth due to its approximately twofold higher egg production rates (10–12 eggs female⁻¹ day⁻¹ compared to approximately 5 eggs female⁻¹ day⁻¹ in D. obtusa) and its shorter juvenile development (3.3 and 2.4 days compared to 6.3 and 5.3 days in D. obtusa); M. brachiata needs generally only 3 molts to reach maturity while D. obtusa requires 5–6 molts.     

Effects of NaCl salinity on the population dynamics of freshwater zooplankton (rotifers and cladocerans)

Salinization of freshwater bodies due to anthropogenic activity is currently a very serious problem in Mexico. One of the consequences may be changes in the rotifer and cladoceran populations, both of which are generally abundant in freshwater bodies. Under laboratory conditions we evaluated the effect of different salt (sodium chloride) concentrations (0–4.5 g l⁻¹) on the population dynamics of ten freshwater zooplankton species (rotifers: Anuraeopsis fissa, Brachionus calyciflorus, B. havanaensis, B. patulus and B. rubens; cladocerans: Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Moina macrocopa and Simocephalus vetulus). All of the zooplankton species tested were adversely affected by 1.5–3.0 g l⁻¹ NaCl. In the range of salt concentrations tested, the population growth curves of B. patulus and B. rubens showed almost no lag phase and reached peak abundances within a week or two; A. fissa had a lag phase of about a week, while both B. calyciflorus and B. havanaensis started to increase in abundance immediately following the initiation of the experiments. Increased NaCl levels reduced the population abundances of A. fissa, B. calyciflorus and B. havanaensis at or beyond 1.5 g l⁻¹. NaCl at 1 g l⁻¹ had little effect on the population growth of cladocerans. M. macrocopa, which was more resistant to NaCl than the other cladoceran species, showed positive population growth even at 4.5 g l⁻¹. The rates of population increase (r, day⁻¹) were generally higher for rotifers than for cladocerans. Depending on the NaCl concentration, the r of rotifers ranged from +0.57 to −0.58 day⁻¹, while the r for cladocerans was lower (+0.34 to −0.22 day⁻¹).